The use of wavelength division multiplexing affords one the opportunity of raising the transmission capacity of a single fiber without the need to develop higher speed components. In switching or networking environments, wavelength division multiplexing arrangements permits optical routing of signals at different wavelengths to different destinations.
Optical sources used in wavelength division multiplexed communications systems must generate light at controlled wavelengths. They require means either to set them permanently to a desired wavelength allocated to a particular channel, or to switch them dynamically to different specific wavelengths for routing purposes. In general, the sources must restrict the wavelengths to prescribed spaced values so that signals do not interfere with each other. Such interference may result from the nonlinear transmission properties of the fiber itself, or from source wavelength drift and insufficient out-off band rejection in the optical filtering or heterodyning technology used to demultiplex the signals at the receiver.
Discrete fixed-frequency sources have been used in wavelength division multiplexing demonstrations. However, using such systems on a commercial basis requires the storage and maintenance of large numbers of reserve sources such as distributed feedback (DFB) lasers. Tunable lasers that attempt to cover the entire range of desired wavelengths have hitherto required cumbersome feedback from external references to provide reliable wavelength stability and often have had insufficient tuning range for many applications.
The paper "A 16.times.1 WDM Transmitter with Integrated DBR Lasers and Electroabsorption Modulators" by M. G. Young et al, paper No. IWA3 in Tech. Digest of 1993 Topical Meeting on Integrated Photonics Research, pp. 414-417, held in Palm Springs, 1993, discloses the use of an array of sixteen independent distributed Bragg reflector (DBR) lasers, each with an integrated electroabsorption modulator, followed by a combiner and an integrated optical booster amplifier, to provide a single fiber port for all 16 wavelength division multiplexed channels. It requires a modulator for each laser. Such an approach demands complex electrical packaging because the arrangement requires a separate high-speed electrical drive capability for each source, either to modulate each laser directly, or to drive each electroabsorption modulator for each laser.
An object of the invention is to improve wavelength-division-multiplexed systems generally.
Another object of the invention is to improve wavelength-division-multiplexed transmitters.
Yet another object of the invention is to overcome the aforementioned difficulties.